(a) Technical Field
The present invention relates to a positive electrode material for a lithium secondary battery and a method for manufacturing the same.
(b) Background Art
With the proliferation of portable electric and electronic devices, the development of improved secondary batteries, such as a nickel hydrogen batteries and lithium secondary batteries, has been actively pursued. In lithium secondary batteries, carbon, such as graphite, is used as a negative electrode active material, an oxide containing lithium is used as a positive electrode active material, and a non-aqueous solvent is used as an electrolyte. Because lithium is a metal which is readily ionized, it is capable of generating high voltages. As such, much work has been focused towards developing lithium secondary batteries having high energy density.
As a positive electrode active material, lithium transition metal oxides containing lithium are widely used in lithium secondary batteries. It is estimated that more than 90% of lithium secondary batteries use layered lithium transition metal oxides, such as lithium cobalt oxide, lithium nickel oxide, and multi-component (cobalt-nickel-manganese ternary system) metal oxide, as positive electrode active materials.
However, when layered lithium transition metal oxides are used as a positive electrode active material, oxygen separates from the lattice structure and undergoes reaction in an abnormal state (such as overcharge and high temperature), thus causing defects such as battery fire. In attempt to address the drawbacks of the layered lithium transition metal oxides, extensive research has been aimed at developing a positive electrode active material having spinel and olivine structures.
In particular, in one method, the use of an olivine lithium metal phosphate as a positive electrode material, instead of the layered lithium transition metal oxide has been proposed. Olivine lithium metal phosphates contain phosphorus as well as oxygen in its main frame which supports its structure, and thus has increased stability as compared to layered oxides. Accordingly, when olivine lithium metal phosphates are used as a positive electrode material, it is possible to provide improved stability.
One such phosphate, lithium iron phosphate (LiFePO4) prepared by coating the surface with 1 to 2 wt % carbon, can provide a capacity of more than 150 mAhg−1, and thus has recently been used in the field of batteries where high safety is required. However, this lithium iron phosphate has a low discharge potential of 3.5 V with respect to lithium, and thus, when it is used in the positive electrode, the discharge potential of the lithium secondary battery is reduced, thereby reducing the energy density of the battery.
In an attempt to address the problem of low energy density due to low discharge potential, International Patent Publication No. WO 2007 034823 and Korean Patent Application Publication No. 10-2008-0047537 describes lithium manganese phosphates having a higher potential. Since the discharge voltage of the lithium manganese phosphate is 4.0 V or higher, it is possible to address the problem of low discharge potential of the lithium iron phosphate. However, the conductivity of lithium manganese phosphates is very low, and thus it is difficult to provide high capacity using material, and the rate capability is significantly reduced.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.